An elastic wave element described in Patent Document 1 includes a piezoelectric substrate, an interdigital electrode provided on the piezoelectric substrate, and a silicon oxide film provided on the interdigital electrode. In order to improve temperature characteristics of the piezoelectric substrate, the silicon oxide film has a smaller thermal expansion coefficient than the piezoelectric substrate, accordingly reducing a thermal expansion of the piezoelectric substrate due to a change of temperature.
The silicon oxide film is formed on the interdigital electrode by a chemical vapor disposition (CVD) method or by a sputtering method. The silicon oxide film formed by the CVD method has a low density. The low density of the silicon oxide film does not affect the propagation of elastic waves having frequencies of several tens MHz having long wavelengths; however, increases a propagation loss for elastic waves having frequencies of several hundreds MHz to several GHz having short wavelengths. The silicon oxide film of the elastic wave element allowing such a high frequency elastic wave to propagate is manufactured by the sputtering method.
The elastic wave element including the silicon oxide film manufactured by the sputtering method may change in its propagation characteristic of the elastic wave according to the change of environmental factor, such as an ambient temperature change.    Patent Document 1: JP2003-209458A